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Fuse keeps blowing

My dad has a heat pump, when the heat is turned on the 3 amp fuse on the 24V red wire off the transformer is blown, a/c is fine. The transformer was replaced, the blower contactor reads 10 ohms across the coil, the backup heat contactor reads 15 ohms. Both work when contactor is pushed in.

Someone told me that one would check all the low voltage wires to see if they are shorted. Does that mean one would disconnect the wire at both ends (at the thermostat being one end) and then check it to ground, hoping to read OL? Since it's the heat not working, either the white or red wire would be shorted, correct?

You need to determine when the fuse blows: on a call for heat or second stage heat, during defrost, or just when the stat is set to "heat". If it's a Rheem product, could be the reversing valve circuit, since they energize in heat. You appear to have eliminated the aux heat control. Likely not the red wire or the fuse would blow in either mode. Check continuity on the "white" wire to common for low resistance. Based on the info you've provided it's the best bet for now.

Update: Someone came to help figure it out and he said it's probably thermostat trouble. We had a new thermostat put in by our local power company for free, which enables them to turn off our power for a short time if needed.

Do you think it could actually be a faulty thermostat? All the wires seem to be hooked up correctly.

Update: Someone came to help figure it out and he said it's probably thermostat trouble. We had a new thermostat put in by our local power company for free, which enables them to turn off our power for a short time if needed.

Do you think it could actually be a faulty thermostat? All the wires seem to be hooked up correctly.

Also for pro membership I think I need 15 posts correct?

I would say that the recent hookup is more than a coincidence to your problem.